1. Field of the Invention
This invention relates to the field of analog signal processing and, more specifically, to circuitry that detects low-level frequency hopping signals in the presence of strong interference signals transmitting very close to the hopping signal's instantaneous frequency.
2. Description of the Related Art
The most widely used method of detecting a frequency hopping low level signal in an adverse environment is through a microprocessor controlled adaptive filter operating in an adaptive noise cancelling circuit. Such a method is disclosed in "Adaptive Noise Canceling: Principles and Application" by Bernard Widrow et al, Proceedings of the IEEE, Vol. 63, No. 12, Dec. 1975. FIG. 1 illustrates the basic concepts of this reference.
As shown in FIG. 1, a signal source S along with a noise source n.sub.o are fed into a modified adaptive noise canceler circuit 1. The noise canceler circuit partially consists of an adaptive filter 25, a power combinet 15 and a power splitter 10 which splits the incoming signal to the adaptive filter 25 and power combinet 15. The incoming signal split by power splitter 10 is then inputed to the adaptive filter 25 which rejects the desired low level signal and passes all other interference frequency components to power combiner 15. The power combinet 15 then combines this interference signal with the incoming signal and outputs only the desired low level signal. The microprocessor 20 compares this output with the expected output to decide if the adaptive filter tap weights need adjustment. To make an adjustment, the microprocessor 20 uses an algorithm that calculates new tap values, by an iteration process, and outputs them to the adaptive filter 25. Thus, the adaptive filter taps are constantly being updated for optimum performance in a dynamic environment. This process, however, has two deficiencies: (1) the microprocessor needs prior information regarding the desired high level signal in order to set the proper tap values, and (2) the algorithm method of calculating tap values is relatively slow, especially when a large number of taps are required to achieve the desired bandpass response.
A modification of this circuit described above is disclosed in U.S. Pat. No. 4,965,581 issued to the inventors herein on Oct. 23, 1990 and entitled "Real-Time Rejection Circuit to Automatically Reject Multiple Interfering Hopping Signals While Passing a Lower Level Desired Signal". This patent is incorporated herein by reference. This patent discloses the use of a chirp-Z analyzer capable of detecting frequency components over a wide bandwidth, with high resolution. In addition to the chirp-Z, the invention utilizes an inverse transform device and a gating circuit. These components, however, significantly reduce the invention's performance and efficiency. The inverse transform device reduces the sensitivity and dynamic range of the invention whereas the gate introduces spikes that cause distortion of the desired signal.
The present invention overcomes the problems described above.